Prehydrolysis of comminuted cellulosic fiber material, such as wood chips, is a well known technique both for ultimately producing pulp or for ultimately producing alcohols. For instance in the conventional prehydrolysis of wood chips before kraft cooking thereof (e.g. see U.S. Pat. No. 3,380,883) the chips are subjected to high temperature (e.g. 170.degree. C.) water, or to a sulfuric acid solution, hydrolysate containing carbohydrate material is removed, and the hydrolysate is conventionally burned to produce steam.
In the conventional production of alcohols, biomass is subjected to acid hydrolysis to break down the cellulose component of the biomass into hexose and pentose sugars with subsequent fermentation of the hexose sugars. Since the same component of the feed material that is utilizable for producing alcohol is under conventional procedures the same component necessary for pulp having good strength properties, the two procedures have conventionally been mutually exclusive. Where prehydrolysis is practiced for pulp production, only dissolving pulp is conventionally produced [dissolving pulp is utilized for the manufacture of rayon or plastics]. Effective production of kraft pulp, suitable for manufacture of paperboard or suitable for oxygen delignification and bleaching to produce paper, has heretofore not been practical. The hydrolysate has usually been removed with the black liquor and burned, and even if not burned (see U.S. Pat. No. 4,174,997) has not been considered suitable for alcohol production. Conversely, when biomass is treated with acid hydrolysis for alcohol production, the biomass residue (as distinct from the hydrolysate) is not suitable for kraft cooking.
According to the present invention, it is possible to treat comminuted cellulosic fiber material so that the carbohydrate material therein is removed in appropriate form to be utilized for the production of alcohols (such as ethanol and butanol), while at the same time the bulk of the comminuted cellulosic fiber material can be subjected to sulfate cooking and can produce a kraft pulp. The pulp produced not only has strength properties within the desired range for the making of paperboard, paper, or the like, but actually has a higher viscosity and higher tear strength than conventional pulps. This is advantageous in that it is possible to effect oxygen bleaching of the pulp to a lower Kappa number with fewer subsequent bleaching stages. Additionally, by practicing the invention it is possible to use a lower active alkali charge in the kraft cooking stage than is conventional.
According to one aspect of the present invention, a method of producing carbohydrate material and kraft pulp from comminuted cellulosic fiber material is provided. The basic steps are: (a) Subjecting the comminuted cellulosic fiber material to mild acid prehydrolysis. The term "mild acid prehydrolysis" as used in the present specification and claims means prehydrolysis treatment sufficient to effect removal of a high percentage of the carbohydrate material (particularly pentosans) from the cellulosic fiber material (alone or in conjunction with the following caustic extraction step to be hereinafter described), while not substantially adversely affecting the cellulosic component of the material, the conditions being an acid concentration of, or equivalent to, about 0.2-0.5% H.sub.2 SO.sub.4, and a temperature of about 120.degree. C. or less. (b) Subjecting the prehydrolysized communited cellulosic fiber material to mild caustic pre-extraction. The term "mild caustic pre-extraction" as used in the present specification and claims means caustic pre-extraction under conditions effective to remove a large percentage of the carbohydrate materials (particularly pentosans) from the comminuted cellulosic fiber material (alone or in conjunction with the previously described mild acid hydrolysis), while not substantially adversely affecting the strength or yield characteristics of the bulk of the cellulosic fiber material which is ultimately subjected to kraft cooking, the caustic extraction conditions being a concentration of, or equivalent to, about 0.5-4% NaOH, at a temperature of about 60.degree.-90.degree. C. (c) Removing hydrolysate having carbohydrate material from the comminuted cellulosic material while practicing the prehydrolysis and pre-extraction steps. The hydrolysate is primarily useful as a feed material for the production of alcohol, and by subjecting it to neutralization, clarification, fermentation, and distillation both the pentose and hexose sugars therein will ultimately be turned into alcohol such as ethanol or butanol. And, (d) effecting kraft (sulfate) cooking of the prehydrolysized, pre-extracted comminuted cellulosic fiber material to produce kraft pulp. The kraft pulp produced has a substantially higher viscosity (and tear strength) than the same pulp produced from the same comminuted cellulosic fiber material and under substantially identical kraft cooking conditions, but without prehydrolysis and pre-extraction.
The method may also comprise the further step of effecting oxygen delignification of the kraft pulp, with ultimate bleaching thereof to the desired brightness. Steps (a) through (d) are practiced so that the delignified kraft pulp produced has a lower Kappa number than such pulp produced from the same comminuted cellulosic fiber material and under substantially identical kraft cooking and oxygen delignification conditions, but without prehydrolysis and pre-extraction. The method is particularly useful where the cellulosic fiber material is chips or sawdust of hardwood, and is desirably practiced so that the prehydrolysis and pre-extraction are effected in a first vessel, and kraft cooking is practiced in a second vessel. The acid prehydrolysis is desirably practiced with a concurrent flow of material and acid, while the caustic extraction is desirably practiced with a countercurrent flow of caustic and material.
The apparatus utilizable in practicing the method of producing carbohydrate material and kraft pulp according to the invention includes many conventional components, such as a chips bin, steamer, high pressure feeder, and continuous digester. The primary addition to the conventional components is the provision of a "first" vertical vessel between the steaming vessel and high pressure feeder on one hand, and the digester on the other. The first vessel includes a material inlet at the top, an acid hydrolysis zone, hydrolysate withdrawal screens, and a countercurrent caustic pre-extraction zone adjacent the bottom. Treated material is fed from the bottom of the first vessel to the top of the digester. Acid for the prehydrolysis may be added in a recirculation loop within the prehydrolysis zone of the first vessel, or can be added at the level tank supplying makeup water to the chips feed to the high pressure feeder. The alkali is added in a recirculation loop associated with the pre-extraction stage, and the recirculating liquid is cooled to maintain the desired pre-extraction temperature (which is normally substantially below the prehydrolysis temperature).
The alcohol production aspects of the present invention are independently utilizable (i.e. even where the production of kraft pulp is not desired or practical), and can maximize alcohol yield from biomass material. For instance where "low quality" (from the pulp production standpoint) cellulosic fiber material is utilized, such as bagesse, after particlization and slurrying the material is fed directly to the hydrolysis-extraction vessel. The vessel in this case is substantially identical to the first vessel of the process and apparatus described above. The acid hydrolysis and caustic extraction steps are practiced at acid and caustic concentrations, and at temperature and residence time conditions, sufficient to effect treatment of the hemicellulose in the biomass material to effect separation of pentose and hexose sugars therefrom into a hydrolysate having insufficient furfural to substantially inhibit fermentation microorganism growth, while not substantially hydrolysizing the cellulose in the biomass. The cellulose component may be washed and dewatered, and subsequently burned for its energy content, while the hydrolysate is removed from the hydrolysized and extracted biomass. The hydrolysate is neutralized and clarified and is subjected to fermentation so that the pentose and hexose sugars therein are fermented. Alcohol is then produced from the fermented hexose and pentose sugars.
The acid hydrolysis and caustic extraction parameters when practicing the method of alcohol production described in the previous paragraph may be slightly different than those utilized in the aspect of the method which results in the production of kraft pulp. For instance the acid concentration can be in the range of about 0.3-2% H.sub.2 SO.sub.4, or the equivalent, and the caustic concentration can be about 1.5-6% NaOH or equivalent. The hydrolysis conditions can be from about 105.degree. C. to 135.degree. C., with about 120.degree. C. being preferred, while the extraction temperature conditions can be from about 60.degree. C.-120.degree. C.
It is the primary object of the present invention to provide for the treatment of comminuted cellulosic fiber material to maximize the removal of suitable carbohydrate material therefrom, while preferably at the same time maintaining the material in condition suitable for the kraft cooking thereof to produce a kraft pulp having satisfactory (and even improved for some parameters) strength properties. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.